The present invention concerns a method for recovering volatile components from excess steam when drying with superheated steam.
In conventional drying processes, for example, convection drying of moist biomass by means of warm or hot air, valuable volatile components such as flavor compounds are lost through the exhaust air.
When storing solid or liquid organic residual materials such as liquid manure or dung, ammonia is lost by evaporation. This is undesirable for several reasons. First of all, ammonia is an important raw material whose synthetic production is very energy-intensive. Secondly, ammonia is an air pollutant and is a health hazard at higher concentrations.
Similar problems arise in case of digestates which are remaining as liquid or solid residues upon fermentation of biomass for biogas production. Ammonium is not decomposed during fermentation. A great proportion of nitrogen of the substrate remains in the digestate. As a result of the decomposition processes, the proportion of the organic soluble nitrogen is shifted toward ammonium so that during anaerobic decomposition the proportion of ammonium will even increase. Ammonium nitrogen is present in aqueous solutions, depending on the pH value and temperature, as ammonium ion or as ammonia. When storing and discharging the digestates there is thus an increased danger of ammonia outgassing. When drying digestates with conventional drying methods, ammonia emissions are also produced.
WO 2009/018997 A1 discloses a method for separation of volatile substances, in particular flavoring compounds. In this connection, the excess steam is passed into a single-stage condenser and is liquefied therein. The resulting liquid is then further passed into an extraction device in order to separate the volatile components from the condensed water. By centrifugal and gravitational forces the light and volatile compounds pass into the gas phase and collect at the upper part of the device. By means of an inert gas, for example, steam (saturated steam), they are entrained and then passed into a condenser. There they are cooled and concentrated. On the other hand, water is thereby moving downward and is separated there.
The thermal water removal from the solid material is realized by drying with superheated water steam in a circulation method at atmospheric pressure. During drying the moisture contents of the solid material passes into the gas phase. This moisture that has been absorbed from the solid material becomes excess steam which is then to be discharged from the drying chamber as a result of volume increase of the gas in the closed circuit.
The invention has the object to provide an improved method for recovery of volatile components from a solid material.